Fibrillation is a particular form of tachycardia that may occur as well in an atrium (atrial fibrillation) as in a ventricle (ventricular fibrillation) of a heart. Other forms of tachycardia are, for example, flutter. A tachycardia is characterized by a rapid heart rate. Typically, fibrillation is characterized by a very high rate of contraction of the heart chamber (atrium or ventricle) affected and of very low amplitude of the sensed electrical potential. Typically, during an episode of fibrillation, no coordinated contraction of the whole heart chamber occurs but only a circulating excitation of the myocardium wherein only one part of the heart chamber's muscle (the myocardium) is excited (depolarised) and thus contracted, whereas other parts of the myocardium already are repolarised and thus relaxed. Therefore, during episodes of fibrillation, the affected heart chamber is unable to efficiently pump blood. For this reason, a ventricular fibrillation (VF) usually is lethal if not treated within minutes or seconds. On the other hand, an atrial fibrillation usually is not life threatening, since the atrial contraction only contributes to a smaller part to the total pumping power of the heart that is typically expressed as an minute volume: pumped blood volume per minute.
A typical treatment of a fibrillation is the delivery of a defibrillation shock to the fibrillating heart chamber. A defibrillation shock usually has a much higher intensity than for example a stimulation or pacing pulse. The intensity of a defibrillation shock shall be sufficient to render the whole myocardium of the fibrillating heart chamber refractory in order to interrupt a circulating excitation of the myocardium and thus to synchronize the contraction of the myocardium of the heart chamber in all it's parts.
Even though atrial fibrillation or atrial flutter is not life threatening, there are several reasons for treating atrial fibrillation, although such treatment is painful for the patient. One reason is that the atrial fibrillation my lead to lethal ventricular fibrillation. Also, atrial fibrillation is compromising the heart's performance because of the loss of atrioventricular synchrony associate with an atrial fibrillation and can cause discomfort as for example, fatigue.
An atrial defibrillation shock for treatment of an atrial fibrillation usually is applied by means of intraatrial shock electrodes. When an atrial defibrillation shock is delivered at the wrong point of time during a heart cycle, the atrial defibrillation itself can cause a ventricular fibrillation. Therefore, delivery of the atrial defibrillation shock during the so-called vulnerable phase of the ventricle is to be avoided. For this reason, it has become common in the art to deliver the atrial defibrillation shock synchronously with a ventricular contraction which also is called a ventricular event. Such ventricular event may be the sensed event in case of a natural (intrinsic) contraction of the ventricle or a paced event, if the contraction of the ventricle is caused by a ventricular stimulation pulse.
U.S. Pat. No. 5,282,836 for example, the disclosure of which is hereby incorporated by reference herein, discloses an atrial defibrillator capable of providing pre-cardioversion pacing. By such pre-cardioversion pacing, the ventricle is stimulated with a fixed pacing rate for a determined number of cardiac cycles in order to establish a stable ventricular rhythm to which the atrial defibrillation shock can be synchronized with.
Further prior art documents dealing with the problem of synchronizing an atrial defibrillation shock with the ventricular rhythm are, for example, U.S. Pat. Nos. 5,282,837 and 6,512,951, the disclosures of which are hereby incorporated by reference herein.